The present invention relates to an AFC (automatic frequency control) apparatus for a satellite broadcasting receiver or the like.
In recent years, a tuning circuit of the frequency synthesizer type using a phase synchronization loop has come to be employed generally for a satellite broadcasting receiver or the like. The use of a frequency synthesizer system stabilizes the local oscillation frequency of a tuning circuit of 1 GHz band with high accuracy. In the case of a satellite broadcasting receiver, however, the local oscillator used for converting a signal of 12 GHz band received by BS antenna into a signal of 1 GHz band in frequency has a frequency drift of about 2 MHz, and therefore an AFC apparatus is indispensable. The satellite broadcasting receivers used in Japan employ an average-value AFC system, in which an average DC voltage of a video signal providing a modulation signal is controlled to a central frequency for modulation.
On the other hand, a high-definition television (Hi-Vision) broadcasting by use of a satellite is planned, and the introduction of the keyed AFC system is considered in Japan as a receiver for such a high-definition television broadcasting.
A satellite broadcasting receiver of the keyed AFC type will be described with reference to the drawings. This receiver is generally configured as shown in FIG. 1. In FIG. 1, reference numeral 1 designates an FM signal input terminal, numeral 2 a mixer for frequency conversion, numeral 3 an FM demodulator, numeral 4 a frequency error detector, numeral 5 a PLL frequency control circuit, and numeral 6 a local oscillator. An example of a general configuration of a satellite broadcasting receiver shown in FIG. 1 also includes an AGC (automatic gain control) circuit and the like which are not related to the function of automatic frequency control and therefore are not shown.
An FM signal of 12 GHz band applied to the input terminal 1 is converted into a signal of intermediate frequency of 1 GHz band by a frequency mixer 2 and applied to the FM demodulator 3. The input signal is FM demodulated by the FM demodulator 3 and reproduced therefrom. The demodulation characteristic of the FM demodulator 3 is generally as shown in FIG. 2. FIG. 2 indicates that with the increase in FM signal frequency, the potential of the demodulation output is increased, so that a frequency change is produced as a voltage change.
The function of the frequency error detector 4 is to decide whether the error of the intermediate frequency from the central frequency (fO) is larger than a predetermined value and whether the frequency error is upward or downward of frequency. In FIG. 2, assuming allowable values of frequency error are (.+-.) .DELTA.f, the output voltage of the FM demodulator 3 may be considered as VH or V.sub.L at the critical allowable value of frequency error.
In view of this, a construction of the frequency error detector 4 as shown in FIG. 3 is considered. Numerals 8 and 9 designate voltage comparator ICs, and numerals 12 and 13 reference voltage sources. As shown in FIG. 3, if the reference potentials are selected at V.sub.H and V.sub.L, the voltage comparator ICs 8 and 9 are turned on at the demodulation output voltages of V.sub.H and V.sub.L respectively, and therefore the outputs AFC1 and AFC2 of the frequency error detector 4 have input-output characteristics as shown in FIG. 4. FIG. 4 shows that if the intermediate frequency becomes lower than the central frequency (f.sub.o) by more than a predetermined frequency error (.DELTA.f), both AFC1 and FAC2 are at low level, and vice versa.
If the oscillation frequency of the local oscillator 6 is controlled by applying these outputs of the frequency error detector 4 to the PLL frequency control circuit 5 in FIG. 1, the intermediate frequency is always controlled within a predetermined frequency error, thereby attaining an AFC function.
A frequency error of about 300 kHz is generally selected for a satellite broadcasting receiver, and if the frequency accuracy of the AFC is to be maintained at a satisfactory level, it is necessary to reduce the temperature drift or the like of the demodulation output of the demodulator to a smaller value.
In the keyed AFC system for the high-definition television broadcasting, a key pulse having a pulse duration as shown in FIG. 5 is produced during the flyback period of a demodulation signal, and while the pulse is produced, the potential of the demodulation signal is kept constant, thus fixing the frequency of the demodulated FM signal. In view of the fact that a frequency corresponding to the potential of a modulation signal during the keyed pulse duration is controlled to a central frequency, the extension of the occupied bandwidth of a demodulation spectrum realized with the change in APL of the video signal is narrow as compared with that for the average value AFC system. As a result, for the purpose of the satellite broadcasting with a fixed transmission bandwidth (DBS), the keyed AFC system is capable of transmitting with a wider degree of modulation.
A configuration as shown in FIG. 6 has thus been generally employed conventionally for the frequency error detector 4 of a keyed AFC apparatus for the satellite broadcasting receiver. In FIG. 6, component parts designated by numerals 7 to 13 have the same configuration as those of corresponding numerals in FIG. 3, in which a demodulation signal is applied to voltage comparator ICs 8 and 9 through a sample hold circuit 17 including an analog switch 15 and a holding capacitor 16. The sample hold circuit 17 applies a key pulse shown in FIG. 5 to a key pulse input terminal 14, and when this pulse is low in level, the analog switch 17 is closed thereby to charge the holding capacitor 16 up to a voltage level of the demodulation output, while the analog switch 15 is opened when the key pulse reaches a high level, thus holding the voltage level of the demodulation output. By comparing this voltage level with reference potentials V.sub.H and V.sub.L at the voltage comparator ICs 8 and 9, frequency error detection outputs AFC1 and AFC2 are produced.
In view of the duty factor of the key pulse as large as about 100 for the high definition television broadcasting system, however, the above-mentioned configuration would cause a change in the level of the demodulated voltage held by the leakage current of the analog switch or voltage comparator IC, thus making accurate comparison by demodulation output values. As a result, the frequency accuracy of frequency error detection is problematically low.